In recent years, increase in plastic waste has become a great social problem. Since many of polymeric materials have hitherto been developed and produced in search of high performance and long-term stability, they are not easily decomposed in a natural environment. Therefore, how to dispose and manage a large quantity of plastic waste which has become useless becomes a social problem on a world-wide scale. The plastic waste includes injection-molded products formed from a variety of synthetic resins, such as polyolefin resins such as polyethylene and polypropylene, polyester resins such as polyethylene terephthalate, polyamide resins such as nylon, chlorine-containing resins such as polyvinylidene chloride, polystyrene, ABS resins, polyacetal, and polycarbonate.
Under the circumstances, biodegradable polymers, which are degraded by natural microorganisms, attract attention as polymeric materials which impose no burden on the environment. The biodegradability can be evaluated by, for example, a degradability test in soil (soil degradability test). Since injection-molded plastic products are required to have good impact resistance, toughness, heat resistance, melt processability, profitability and the like, however, any injection-molded plastic product, which fully satisfies these requirements and exhibits biodegradability, has not been yet obtained.
Among the conventional biodegradable injection-molded plastic products, for example, injection-molded products based on starch are unsatisfactory in toughness and heat resistance and involve a problem that such a plastic material is difficult to injection-mold because it is non-crystallizable. Injection-molded product based on cellulose are unsatisfactory in impact resistance, toughness and heat resistance and involve a problem that such a plastic material is difficult to injection-mold because it is low in crystallizability. Injection-molded products based on a microorganism-produced polyester involve a great problem that their production cost is very high. Injection-molded products based on a synthetic type polyester such as a polysuccinate are unsatisfactory in impact resistance, toughness and heat resistance and involve a problem that succinic acid and butanediol, which are raw materials for the polyester, are considerably expensive in addition to the fact that such a plastic material is unsatisfactory in injection moldability.
Injection-molded products based on polylactic acid, which is a semi-synthetic type polyester, must be produced through fermentation of a biological process because L-lactic acid, which is an optically active substance used as a raw material, is required to have a high purity. There is hence a limit to their production at low cost. Further, since polylactic acid has a high glass transition temperature, Tg, it also involves a problem that it is difficult to compost under ordinary composting conditions.